必须 (bì xū) is an essential modal verb meaning 'must' or 'have to', expressing strong necessity or obligation. It combines 必 (bì), meaning 'certainly' or 'inevitably', and 须 (xū), meaning 'must' or 'need to' — together they reinforce the idea of non-negotiable requirement. Unlike English 'must', 必须 always precedes the main verb and cannot stand alone; it functions as an adverb modifying the verb that follows.

This word appears frequently in formal instructions, rules, safety guidelines, academic requirements, and daily responsibilities — for example, in school policies ('Students must submit assignments on time') or health advice ('You must wash your hands before eating'). It carries a tone of authority or urgency and is rarely used in casual, friendly suggestions (where 应该 or 得 is preferred). While similar to other necessity markers, 必须 implies the strongest level of compulsion, often backed by external rules or serious consequences.